Plug - Lightweight OSX player for Hype Machine
Poolsuite™ - The super-summer music player
Rocketgraph - Rocketgraph is an open-source drop in replacement to Firebase, except it's much better.
Spotify - Map shows when two people play same song at same time
Muru Music - Personalised playlists within seconds.
Playlist Hunt - Playlist Hunt is one of the best applications for Apple users to get the best music through it.